A standard rail-transportation system has two horizontally parallel rails on which ride a plurality of cars that are joined together into trains and pulled or pushed by an engine. Such a system is enormously expensive, and due to the necessity of having to make up trains does not give much flexibility of use.
A one-rail cog railway is also known which has one large rail provided on its underside with a rack that meshes with a gear carried on an engine that pushes or pulls a train of cars that ride on the rail. This system is particularly useful in cold mountainous terrain, as the positive engagement between cog gear and rack allows considerable tractive effort to be employed, and all the vital drive parts are hidden out of harm's way under the rail where snow and ice cannot foul them. This system, nonetheless, is quite expensive to build and requires the use of trains which inherently reduces the frequency of trips.
Various other systems are known, such as overhead monorails, which can only be built at enormous expense, and overhead suspended railways whose rails are hung from catenary suspension cables, which are experimental and of dubious safety.
Cable railways where the individual cars ride on their own tracks and are clamped to normally underground cables are known, but have proven impractical. Such systems usually lose a large percentage of their energy in displacement of the traction cable which obviously is a large item that cannot be kept moving over its various guides and rollers without considerable loss of energy to friction.